Formation damage due to invasion by drilling fluids is a well known problem. Many zones contain formation clays which hydrate when in contact with water such as the filtrate from drilling fluids. These hydrated clays tend to block the producing zones, primarily sands so that oil and gas cannot move to the borehole and be produced.
These zones are also damaged by solids which are carried into the openings with the fluid. The movement of drilling fluids and filtrate through these openings also causes the dislodging and migration of solids in place in the formation. These solids can lodge and block movement of produced hydrocarbons.
Invasion is caused by the differential pressure of the hydrostatic column which is generally greater than the formation pressure, especially in low pressure or depleted zones. Invasion is also due to the openings in the rock and the ability of fluids to move through the rock, the porosity and permeability of the zone.
Because of this differential pressure, drillers have long used filtrate control mechanisms to control the movement of drilling fluids and filtrate in to and through the formation openings. This mechanism involves adding particles to the drilling fluid which are then deposited onto the borehole wall while circulating and drilling. These particles are generally some combination of bentonite, starch, lignins, polymers, barite, and drilled solids. They are used to plug and seal the borehole due to the particle size and shape, and some control is also due to the viscosity of the filtrate when water soluble polymers are used. Although this wallcake forms a semipermeable barrier, some filtrate moves through and into the zone both before and after the wallcake is formed.
Wallcake control, then is not complete and some filtrate water is allowed to contact the producing zone. Another disadvantage of wallcake mud is that when filtrate moves through, the solids are screened out and left in the cake. This causes the cake to become thicker and can lead to differential sticking of the drill string.
More recent technology has seen the development of Low Shear Rate Viscosity (LSRV) fluids. LSRV is created by the addition of specialized polymers to water or brines to form a drilling fluid. These polymers have a unique ability to create extremely high viscosity at very low shear rates. These LSRV fluids have been widely used because of their carrying capacity and solids suspension ability. They have been accepted as a way to minimize cuttings bed formation in high angle and horizontal wells, and as a way to reduce barite sag in high weight muds.
Recent studies and field experience indicate that this LSRV is helpful in controlling the invasion of drilling fluids and filtrate by creating a high resistance to movement into the formation openings. Since the fluid moves at a very slow rate, viscosity becomes very high, and the drilling fluid is contained within the borehole with a very slight penetration. This has been beneficial in protecting the zones from damage as well as reducing differential sticking in these fluids. See for example the article entitled "Drill-In Fluids Improve High Angle Well Production", Supplement to the Petroleum Engineer International, March, 1995.
Lost circulation is also a severe problem in rotary drilling. Lost circulation occurs when the differential pressure of the hydrostatic column is much greater than formation pressure. The openings in the rock are able to accept and store drilling fluid so that none is returned to surface for recirculation. The fluid is lost downhole and can become an expensive and dangerous problem. Lost circulation can lead to hole instability, stuck drill pipe, and loss of well control. At the least, it halts drilling operations and requires expensive replacement volume to be used.
In addition to the fluid volume being lost, expensive lost circulation materials (LCM) are required. These are usually fibrous, granular, or flake materials such as can fibers, wood fibers, cottonseed hulls, nut hulls, mica, cellophane, and many other materials. These LCM materials are added to the fluid system so that they may be carried into the loss zone and lodge to form a bridge on which other materials may begin to build and seal. These LCM materials themselves are damaging to the zones, and because they must be carried many times in the drilling fluid to maintain circulation, solids removal is halted and high solids mud results.
Methods of correcting lost circulation of drilling fluids by aerating the drilling fluids are set forth in U.S. Pat. No. 2,818,230 (Davis) and U.S. Pat. No. 4,155,410 (Jackson).
The use of underbalanced drilling has increased as the development of low pressure formations has acquired more importance. Horizontal drilling, in particular, has increased the need to drill across zones that are not only low pressure, but highly fractured or permeable. The exposure of numerous fractures or openings having low formation pressures has increased the problem of lost circulation and formation invasion. The necessity of down hole tools many times preclude the use of bridging materials to stop these losses. This has led to the use of underbalanced drilling techniques to control the losses and invasion of these zones. Some of these techniques include the use of air, mist, and foam drilling fluids. Problems with these fluids include hole cleaning, control of formation fluids, corrosion, and requirements for expensive, often hard to get equipment such as compressors and boosters. Such fluids are not re-circulateable and must be constantly generated as the drilling proceeds.